The present invention concerns signal test and measurement and pertains particularly to alignment self check for a wavelength meter.
In a Michelson interferometer system, light from a fiber optic input is collimated and directed to the input of the interferometer. The light from a fiber optic input is split into two paths with a beam splitter to form two beams. Both beams are then reflected by mirrors that redirect the light back toward the beam splitter. One portion of the light reflected from the mirrors goes back toward the input of the interferometer. The other portion of the light is incident on a photodetector. Assuming there is no loss in the interferometer, all of the light is directed to either the photodetector or toward the input of the interferometer.
One mirror of the interferometer is stationary and one is movable. The movable mirror varies the length that the beam travels before and after incidence with the movable mirror. As the mirror is moved, the amount of light reaching the photodetector oscillates up and down because of constructive and destructive interference effects between the two paths of the interferometer. Through the analysis of these interference patterns, the wavelength of light can be calculated.
The beams of light can be analyzed in terms of light interfering as the path length in the interferometer changes. This is referred to as the fringe-counting description of wavelength meter operation. Alternately, if the movable mirror is moved at a constant rate, the frequency of the light in the beam is Doppler-frequency shifted. A Doppler detector then mixes the light from the moveable mirror and the stationary mirror. A resulting beat frequency between these two signals can be used to calculate the unknown frequency of the input signal. See Dennis Derickson, Fiber Optic Test and Measurement, Prentice Hall, Inc., 1998, pp. 133–141.
A Michelson interferometer based wavelength meter measures the wavelength of an unknown signal by comparing a fringe pattern produced by the unknown signal with that of the reference (known) signal. As one arm of the Michelson interferometer is translated (i.e., the mirror is moved) the interference pattern at the photodetector oscillates between high and low irradiance. Comparing the number of fringes produced by the unknown signal with the number produced by the known signal results in a highly accurate estimate of the unknown wavelength. The accuracy of the measurement can be further increased by multiplying the frequency of the fringes electronically with a phase locked loop.
Unfortunately, the measurement accuracy of a Michelson interferometer can be significantly reduced if the unknown signal and known signal are not aligned with one other. The alignment of the interferometer can degrade over time due to shock, vibration and stress due to thermal expansion.